The present invention relates to a melt-processable tetrafluoroethylene copolymer having excellent processability and thermal stability, and more particularly to a thermally stable tetrafluoroethylene copolymer which is obtained by stabilizing thermally unstable groups existing at the copolymer ends, and a process for preparing the same.
Copolymers prepared from tetrafluoroethylene and perfluoro(alkyl vinyl ether) (hereinafter referred to as "PFA") have been well known as a melt-processable fluorocarbon resin, and have been widely used for various uses, for instance, as starting materials for molded articles such as tubes, pipes, joints, containers and coating materials for electric wire, as coating materials, as lining materials, as starting materials for roto-molded articles such as hollow articles, and the like.
As to PFA, more or less amount of group --COF is inevitably formed at the polymer ends due to the polymerization mechanism of PFA. Also, in case of emulsion polymerization using a polymerization initiator such as ammonium persulfate (APS), groups --COOH are produced at the polymer ends, or in case of using methanol as a molecular weight controlling agent, groups --CH.sub.2 OH or groups --COOCH.sub.3 are formed at the polymer ends. Since these terminal groups are thermally unstable, it has been known that the bubble formation during molding or the generation of fluorine-containing acids is caused from these terminal groups, thus resulting in failure of molding or corrosion of a mold of a molding machine.
Under the circumstances, a technique to stabilize these thermally unstable terminal groups has hitherto been studied and some have been proposed. For instance, Japanese Unexamined Patent Publication No. 61-98709 discloses a process for preparing a thermally stable PFA which comprises contacting PFA with ammonia gas or a nitrogen compound capable of producing ammonia for a time sufficient to convert at least 50% of terminal groups --COF and --COOH into groups --CONH.sub.2.
Also, Japanese Unexamined Patent Publication No. 62-104822 discloses a method wherein PFA having more than 6, per 10.sup.6 carbon atoms, of terminal groups --CF.sub.2 CH.sub.2 OH, --CONH.sub.2 or --COF is contacted with a fluorine-containing gas under temperature, time and pressure conditions sufficient to decrease the number of the terminal groups to less than 6 per 10.sup.6 carbon atoms.
However, when PFA prepared through the process for stabilizing the thermally unstable terminal groups according to the above-mentioned publications is subjected to the roto-molding, the obtained molded article is greatly shrunk by heating or is poor in dimensional stability on heating. Also, according to the fluorinating method disclosed in Japanese Unexamined Patent Publication No. 62-104822, it is required to conduct the reaction at a high temperature for many hours in order to heighten the conversion for fluorination. If the severe reaction conditions can be lightened even a little, such a reaction is advantageous from the viewpoints of the economy and the safety.
An object of the present invention is to provide a thermally stable PFA having excellent powder properties, which can provide a molded article having no bubbles and excellent dimensional stability.
A further object of the present invention is to provide a preparation process of the thermally stable PFA as mentioned above.
These and other objects of the present invention will become apparent from the description hereinafter.